Notices of Memoirs — Dr. R. H. Traquair's Address. 521 



•more reduced, having become very small and short, with only the 

 axis remaining. 



From this point of view, then, Dipterns, instead of being the most 

 ■specialized Dipnoan, is the most archjBic, and the modern Ceratodus, 

 Frotopteriis, and Lepidosiren are degenerate forms ; and instead of 

 the Crossopterygii being the offspring of Dipterus-\\\iQ forms, it is 

 exactly the other way, the Dipnoi owing their origin to Holoptychiidse, 

 which again are a specialization on the Khizodontidse, though they 

 did not survive so long as these in geological time. Consequently 

 tlie Ceratodus limb, with its long median segmented axis and biserial 

 arrangement of radials, is not an archipterygium in the literal sense 

 of the word, but a derivative form traceable to the short uniserial 

 type in the Rhizodonts. But from what form of fin that was derived 

 is a question to which palaeontology gives us no answer, for the 

 progenitors of the Crossopterygii ai'e as yet unknown to us. 



Plausible and attractive as this theor^'^ undoubtedly is, and though 

 it relieves the palaeontologist from many difficulties which force 

 themselves upon his mind if he tries to abide by the belief that the 

 Dipnoan form of limb had a selachian origin, and was in turn handed 

 on by them to the Crossopterygii, yet it is not without its own 

 stumbling-blocks. 



First as to the dentition, on which, however, M. Dollo does not 

 seem to put much stress, it is impossible to derive Dipterns directly 

 from the Holoptychiidae, unless it suddenly acquired, as so many of 

 us have to do as we grow older, a new set of teeth. The dendrodont 

 dentition of Holoptychius could not in any way be transformed into 

 the ctenodont or ceratodont one of Dipterns : both are highly 

 specialized conditions, but in different directions. Seraon has 

 recently shown that the tooth-plates of the recent Ceratodus arise 

 from the concrescence of numerous small simple conical teeth, at first 

 separate from each other.^ Now this stage in the embryo of the 

 recent form represents to some extent the condition in the 

 Uronemidaj of the Carboniferous and Lower Permian, which stand 

 quite in the middle of Dollo's series. 



Again, the idea of the origin of the Dipnoi from the Crossopterygii 

 in the manner sketched above cuts off ever}'^ thought of a genetic 

 connection between the biserial archipterygium in them and in the 

 Pleuracanthidae, so that we should have to believe that this very 

 peculiar t^'^pe of limb arose independently in the Selachii as a 

 parallel development. It may be asked, Why not? We may feel 

 perfectly assured that the autostylic condition of the skull in the 

 Holocephali arose independently of that in the Dipnoi, as did like- 

 wise a certain amount of resemblance in their dentition. But those 

 who from embryological grounds oppose any notion of the origin of 

 the Dipnoi from ' Ganoids ' might here say, if they chose, If so, 

 why should not also the same form of limb have been independently 

 evolved in Crossopterygii ? 



Accordingly, while philosophic palaeontology is much indebted to 

 M. Dollo for his brilliant essay, and though we must agree with him 

 1 ''Di^Zahnentwickelung des Ceratodus Forsteri^\; Jena, 1899 



